>Has anyone noticed the tubes goingYes, absolutely. I might have to pull mine out a tweak it a bit based on your observations.
>into cutoff with larger input signals?
Tony
To: cgs_synth@yahoogroups.com
From: synthlist@...
Date: Mon, 7 Jul 2008 22:02:38 -0500
Subject: [cgs_synth] 1j17b tube vca becomes vcf
hi all,
i built a tube vcf this last week using 1j17b. this is heavily based
on ken's tube vca idea. it is a sallen-key using the tube for gain and
vactrols control the cutoff frequency. i still have opamps at the
input and output as in ken's circuit.
a couple of questions and comments: Has anyone noticed the tubes going
into cutoff with larger input signals? I built the amp as ken's
circuit twice with 3 different 1j17b's and got the same result. the
problem was that as the input increased, the voltage at the control
grid decreased until the tube cutoff. it decreased a few volts. i was
looking at average voltage of -18 or -19V at the grid with a 10v p-p
input. mine only accepted very small inputs without clipping most of
the signal. looking at my archived email, it seems one other person
has had this problem.
I ended up getting the output more centered with soft clipping on both
the high and low sides of the amplified signal by making a few changes
(i tried very many resistor combinations plus a few circuit
configurations, and i tested this on two 1j17bs in two separate
builds). note, i set this for a gain of about 3 when a full 15v is at
the screen input. also note: i knew nothing about tubes a week ago,
and now i know only a tiny amount.
-connect the 330k grid resistor to -15v as one option allows.
-set anode (plate) resistor to 27K intead of 8.2K. I did this for some
gain, but it is balanced well with my other settings. i didn't notice
any terrible distortion even with 47K. i never got much gain with the
8.2k resistor. I expect this was mostly due to the tube going into
cutoff.
-set the tube's input resistor to 330K. this was 2.2K. i used a 330K.
with that, the grid current will cause it to clip at the on the
positive side of the input signal which means the negative side of the
output is clipping. the clipping is fairly soft on that side though.
-set screen resistor to 27K. I found this helped even things out also.
It seemed that there was still a tendency to clip at the positive side
of the output before the negative side until i did this.
now, it is like this: the positive side of the input signal, and
positive side of the output signal are soft clipping when the input is
large (+-5V is very large). since its inverting, the output will be
roughly symmetrical. note, there is a fair bit of distortion except
for at the smallest input signals (+-1 or 2V). no matter the input
signal, the tube limits the output to about 15Vp-p i guess.
so, for the filter version. it sounds nice i think. the thing will
resonate fine but requires some input to get it started. I have been
testing and i'm pretty happy with it. I'll be preparing a PCB layout
this week, and i'll be trying with some 1j24b's soon. i'll post some
schematics after that. here is a sample of its sound. it starts off
high resonance, low input. then high input high resonance. then high
input lower resonance.
http://www.homebuilthardware.com/tmp/sk-tube-vcf.mp3
for anyone interested there is a really nice article here:
http://www.freewebs.com/valvewizard1/Common_Gain_Stage.pdf
which describes these two types of tube clipping very well. i highly
recommend reading it!
-Ryan
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